Back on topic: Keep in mind that in OHKOing Gengar, you're not there yet. You won't OHKO on turn 1 (and likely not on turn 2 either), so Gengar can easily get ahead and disrupt you while working to its win condition. What are you going to do about THAT?
I don't know, I did a bit of testing last night and Weavile is pretty easy to get to OHKO status or at least OHKO + Flash Bite status on T2. If you start Weavile you attach and Call For Family, 2nd turn attach again and attach E Gain. If you don't start with him, you attach Call to whoever you start with and Call for Bronzong and Weavile, 2nd turn Galactic Switch, attach, and Egain. I wasn't able to pull it off every game, but usually I could.
I'm not worried about Gengar getting more than 1 energy on him. I'm not going to have any Pokemon in my hand unless they Seeker, and then it will only be one. And they can't Seeker every turn if I'm OHKOing Gengar because they'll have to Twins to stay on track. Also, Spiritomb TM and PGX have forced my Spray count up to 4, which I'm actually really starting to like in other match-ups as well.